Recent years have seen technical advances in portable electronic devices and hybrid vehicles etc., and there has been a growing demand for secondary batteries with a higher capacity (in particular, lithium-ion secondary batteries with a higher capacity) for use in those devices and vehicles. The development of high-capacity cathodes for lithium-ion secondary batteries currently lags behind that of high-capacity anodes. Even actively researched and developed high-capacity Li(Ni,Mn,Co)O2-based materials only have a capacity of about 250 to 300 mAh/g.
Sulfur, which has a theoretical capacity of as high as about 1,670 mAh/g, is one of the promising high-capacity electrode materials. However, elemental sulfur alone does not contain lithium, and lithium or a lithium-containing alloy is thus required for use in the anode, leaving few options for the anode.
Lithium sulfide, on the other hand, contains lithium, and thus allows alloys containing graphite or silicon, for example, to be used in anodes. Thus, lithium sulfide can provide a considerably wider selection of anodes, and this prevents risks such as short-circuit caused by dendrites generated by metal lithium. However, lithium sulfide, as lithium polysulfide, dissolves into an organic electrolyte during charge or discharge in a battery system that contains the organic electrolyte, and migrates into the anode to cause segregation (e.g., NPL 1), making it difficult to extract the inherent high capacity of lithium sulfide. To improve the performance of batteries that include lithium sulfide in a cathode, some measures are needed, such as, designing cathode layers capable of retaining the dissolved lithium polysulfide in the cathode; creating electrolytes capable of protecting the anode; and providing alternative solid electrolytes not involving lithium polysulfide flow.
One method for reducing the dissolution and flow of lithium polysulfide is to form bonds between sulfur atoms and other elements so that sulfur atoms cannot be released during Li extraction and insertion reaction. Doing this requires the preparation of a compound of lithium sulfide to which other elements are incorporated. “Other elements” suitable for incorporation are transition metal elements that can impart electrical conductivity to the insulating lithium sulfide, and examples include compounds such as LixFeySz disclosed in PTL 1. However, adding a large amount of other elements increases the chemical formula weight of the electrode active material, and also reduces the relative Li content, decreasing the theoretical capacity. In PTL 1, for example, an equimolar amount of FeS2 is added to Li2S to form a composite, and the Fe content is thus 17% and the Li content is 33%, with the theoretical capacity estimated from the Li content being about 320 mAh/g, which is significantly lower than the theoretical capacity of lithium sulfide (about 1,170 mAh/g). Therefore, the amounts of other elements added must be minimized in the production of high-capacity electrode materials.
However, decreases in the amount of other elements increase free sulfur, which means an increase in the proportion of sulfur atoms that do not contribute to the charge and discharge reaction. The use of transition metals as other elements to be added not only further decreases the electrical conductivity, but also decreases the utilization rate of the electrode material. For example, as disclosed in NPL 2, a decrease in the Fe content of a Li2S—FeS2 composite from 16% to 3% increases the theoretical capacity from about 350 mAh/g to about 930 mAh/g, but the capacity obtained in the actual charge and discharge decreases from about 250 mAh/g to about 3 mAh/g. Because about 10% or less of the Fe content is considered to be sufficient to form Fe—S bonds and to impart electrical conductivity, the probable reason for the decrease in actual measured capacity is that the added Fe atoms are incorporated into the lithium sulfide crystal lattice, and fail to form Fe—S bonds. More specifically, lithium sulfide itself remains largely unchanged in the process of forming a composite, and the incorporated Fe atoms are present as a byproduct of Li2FeS2 without contributing to an increase in the utilization rate of the composite. This occurs because the composite is prepared only by heating treatment, and it is difficult to incorporate other elements into the lithium sulfide crystal lattice to form Fe—S bonds by heating treatment alone. Adding carbon can make the composite electrically conductive, but it is difficult to form C—S bonds to reduce free sulfur atoms only by heating treatment. Thus, transition metal elements must be added.
Examples of means to solve the problems include a method comprising incorporating Fe atoms into the lithium sulfide crystalline structure by a combination of heating treatment and mechanical milling. NPL 3, for example, discloses a method for preparing a LixFeySz—C composite by heating Li2S and FeS2 (or FeS), then mixing it with about 10% of carbon powder, and subjecting the mixture to mechanical milling for 8 hours using a grinder mill. NPL 3 reports that even when the Fe content is 2 to 10 atomic %, the composite exhibits a high capacity.